I.1. Definitions
A fingerprint is a particular pattern consisting of lines corresponding to the ridges and valleys appearing on the fingers, palms and soles. Since Bertillon's studies at the beginning of this century, it is well known that fingerprints contain specific features, called minutiae, which are unique and allow identification of people by their fingerprints. By definition, a minutia is either (1) a bifurcation, which is the location where a given line forks into two different lines (FIG. 4), or (2) a ridge ending, which is the location where a given line ends (FIG. 5). Minutiae are usually recorded with three coordinates: two coordinates "x" and "y" for the position of the minutia relative to a coordinate system, and one coordinate "a", which is an angle representing the average direction of the lines around the minutia point. (See FIGS. 4, 5).
The automatic "matching" of fingerprints under the control of a general purpose computer is the process of comparing two different fingerprints to determine if they come from the same finger, and thus from the same person. The automatic matching of two fingerprints utilizes a number, known as the "matching score" to indicate the degree to which two given fingerprints look alike; the greater the score, the greater the probability that the two fingerprints come from the same finger.
The automatic "identification" of a fingerprint under the control of a general purpose computer is the process of comparing a given fingerprint, called a "search print," to a database containing a set of fingerprints called "file prints," in order to determine whether the database contains a file print which comes from the same individual as the search print. Thus, if the database contains N file prints, one identification is equivalent to N matchings. The comparison of the search print against each of the N file prints produces N matching scores, which are sorted by decreasing value. Identification typically results in a short list of "candidates," which are the file prints which have produced the best matching scores. The accuracy of an automatic system for matching and identifying fingerprints is measured by its ability to place the right print in the list of top candidates.
The principal sources of fingerprint data for such database are (1) "tenprint cards", which typically contain the inkings for all ten fingers of a given individual, (2) latent prints, which are one or more parts of fingerprints left, e.g., at the scene of a crime and (3) fingerprint images obtained "live" by means of an optical apparatus for reading fingerprints directly from the hand.
A tenprint card generally includes alphanumeric data, such as name and age, and ten inked rolled fingerprints. The fingerprints are grouped in two sets, one for each hand. An approximate orientation of the prints on each card is known, since the fingerjoint is supposed to be parallel to the card's base line for the five fingerprints from the same hand.
By contrast, the orientation for latent prints is generally not known, since only a part of the fingerprint is available. Furthermore, latent prints are often of very poor image quality. As a result, the problem of accurately matching and identifying latent prints has long stood as a major problem in the art of automatic fingerprint identification systems.